(a) Describe Bohr's model of the atom. How did Neils Bohr explain the stability of atom?

(b) An element has an atomic number of 11 and its mass number is 23. What is the arrangement of electrons in the shells? State nuclear composition of an atom of the element.

(a) Bohr's model of atom

An atom is made up of three particles: electrons, protons, and neutrons.

(i) The protons and neutrons are located in a small nucleus at the centre of the atom.
(ii) The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells.
(iii) There is a limit to the number of electrons that each energy level can hold.
(iv) Each energy level is associated with a fixed amount of energy.
(v) There is no change in the energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable.

Niels Bohr explained the stability of the atoms through the concept of revolution of electrons in different energy levels. The change in the energy of an electron occurs when it jumps from a lower energy level to a higher energy level or vice versa. When it gains energy, it jumps from a lower energy level to a higher energy level, and when it loses energy it comes down from a higher energy level to a lower energy level. In this way, energy is not lost and the atom remains stable.


A few energy levels in an atom


(b).Given: Atomic no. = 11
Mass no. = 23,
Electronic configuration – (2, 8, 1)

Nuclear composition of an atom of the element is – 11 protons and 12 neutrons